In addition, students already considered popular engage in these forms of bullying, the researchers found.

The UCLA psychologists studied 1,895 ethnically diverse students from 99 classes at 11 Los Angeles middle schools. They conducted surveys at three points: during the spring of seventh grade, the fall of eighth grade and the spring of eighth grade.

Each time, students were asked to name the students who were considered the "coolest," the students who "start fights or push other kids around" and the ones who "spread nasty rumours about other kids."

Those students who were named the coolest at one time were largely named the most aggressive the next time, and those considered the most aggressive were significantly more likely to be named the coolest the next time.

The results indicate that both physical aggression and spreading rumours are rewarded by middle school peers.

"The ones who are cool bully more, and the ones who bully more are seen as cool," Jaana Juvonen, lead author of the study, said.

"What was particularly interesting was that the form of aggression, whether highly visible and clearlyconfrontational or not, did not matter. Pushing or shoving and gossiping worked the same for boys and girls.

"The impetus for the study was to figure out whether aggression promotes social status, or whether those who are perceived as popular abuse their social power and prestige by putting other kids down.

"We found it works both ways for both 'male-typed' and 'female-typed' forms of aggression," she said.

The study implies that anti-bullying programs have to be sophisticated and subtle to succeed.

"A simple message, such as 'Bullying is not tolerated,' is not likely to be very effective," Juvonen said.

Effective anti-bullying programs need to focus on the bystanders, who play a critical role and can either encourage or discourage bullying, said Juvonen, who has conducted research on bullying since the mid-1990s and serves as a consultant to schools on anti-bullying programs.

Bystanders should be made aware of the consequences of spreading rumours and encouraging aggression and the damage bullying creates, she said. The study has been published online in the Journal of Youth and Adolescence.